Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, toner particles are given a charge substantially opposite to the charge of the latent image, and brought into the vicinity of the photoreceptor so as to be attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g. clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g. of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic (EP) printers typically transport the receiver past the photoreceptor to form the print image. The direction of travel of the receiver is referred to as the slow-scan or process direction. This is typically the vertical (Y) direction of a portrait-oriented receiver. The direction perpendicular to the slow-scan direction is referred to as the fast-scan or cross-process direction, and is typically the horizontal (X) direction of a portrait-oriented receiver. “Scan” does not imply that any components are moving or scanning across the receiver; the terminology is conventional in the art.
Various EP printers, especially low-cost, low- to medium-volume, cartridge-based printers using mono-component toner exhibit banding artifacts in low-frequency areas such as sky. These artifacts appear in a repeating pattern, and extend in the fast-scan direction.
One scheme to reduce banding is to separate supply and toning members in the printer when not printing. However, this adds cost and complexity to the printer. Additionally, this scheme requires maintaining precise alignment and consistent pressure between and across the lengths of the supply and toning members over many cycles of printing and idling between print jobs.
Another scheme to reduce banding is to rotate the supply and toning members on a fixed schedule. However, over time, this causes unnecessary wear on the supply and toning members and the photoreceptor and consumes energy. Moreover, this scheme causes increased levels of toner stress, which itself exacerbates the appearance of banding artifacts. “Toner stress” is physical and electrical changes to the toner particles, which changes are caused by repeated physical contact and mechanical pressure applied to the toner particles such as being repeatedly removed and returned to the toner supply container. For example, toner particles deform and fracture, and surface treatments, designed to enhance and regulate electrical charging, wear off. All of these factors reduce image quality.
There is a continuing need, therefore, for a way of reducing banding artifacts that maintains image quality and low cost, and reduces toner stress.